1. Field of the Invention
The present invention relates to a non-reciprocal circuit device and, in particular, to a non-reciprocal circuit device, such as an isolator or a circulator, used in microwave bands.
2. Description of the Related Art
A non-reciprocal circuit device, such as an isolator or a circulator, has characteristics that allow transmission of a signal in a predetermined direction but not in a reverse direction. Because of these characteristics, for example, an isolator is used in a transmitter circuit of a mobile communication device, such as an automobile telephone or a cellular phone, for example.
To reduce insertion loss, International Publication No. 2007/046229 describes a 2-port isolator that includes a first central electrode and a second central electrode that arranged on the ferrite so as to cross each other and so as to be electrically insulated from each other. A terminating resistor, which is arranged in parallel with the first central electrode and connected between an input port and an output port, is built in the circuit board. High frequency signals traveling in a reverse direction generate heat that is dissipated at the terminating resistor. If the terminating resistor does not adequately radiate heat, the electrical characteristics of the isolator deteriorate due to the increased temperature. Therefore, in order to avoid overheating, the terminating resistor must adequately radiate heat.
A non-reciprocal circuit device disclosed Japanese Patent 4003650 addresses the heat radiation of a terminating resistor. The non-reciprocal circuit device according to Japanese Patent 4003650 ensures adequate heat radiation by providing a via-hole in the dielectric substrate. Conventionally, improving a power handling capability by improving the heat radiation ability of the terminating resistor primarily depends on the power consumption of the terminating resistor. However, in the non-reciprocal circuit device disclosed in Japanese Patent 4003650 heat is still primarily generated at the terminating resistor.